Glideslope Capture From Above
You are pilot flying a twin-jet on the ILS RWY 23L approach at Auckland in IMC and moderate turbulence. You were kept at 7,000 ft for traffic and are cleared for the approach from 10 DME, well above the published profile. At 8.0 DME the glideslope pointer comes off the top of the scale and centres; the autopilot captures it and the aircraft starts down. At 6.5 DME you are passing 6,000 ft with a required rate of descent near 3,200 ft/min at your 200 kt groundspeed. The localiser is centred, the DME is working, and the approach chart's 3.00° path would have you at about 2,090 ft at that point. The PM notes that the last 500 ft of descent took less than 10 seconds and that the GS pointer has stayed centred throughout.
1. Explain what has happened, using the geometry and the rate of descent to support your conclusion.
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You have captured a false glideslope lobe. A glidepath transmitter produces the main path at 3.00° and image lobes at roughly three times the basic angle (about 9°), five times (15°) and so on, formed by ground reflections and the antenna pattern. A receiver can lock onto one of these if the aircraft is already high when the GS becomes usable.
The geometry confirms it. On the true 3.00° path you would be at about 2,090 ft at 6.5 DME (6.5 × 6,076 × tan 3° ≈ 2,070 ft plus 23 ft threshold elevation). A 9° path at the same DME is about 6,280 ft, so being at 6,000 ft with a centred pointer is consistent with the 9° lobe, not the 3° path.
The rate of descent confirms it a second time: a 3° path at 200 kt ground speed needs about 1,060 ft/min, while your PFD demands roughly 3,200 ft/min. That is three times a normal ILS rate, well outside any stable approach criteria, and is itself a hard warning. The localiser is unaffected - only the vertical guidance is invalid.
2. What is the correct action, and how do you re-establish the approach?
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Reject the glideslope. The approach cannot be continued from above the path, and following a 9° lobe would put you in an unstabilised descent at about 3,200 ft/min with no assurance of obstacle clearance.
- Deselect the approach mode or disconnect the autopilot so the aircraft does not continue down; level off at a safe altitude.
- Tell ATC you have an invalid glideslope capture and request either vectors to re-intercept from below at the published intercept altitude, or a hold to lose height.
- Re-intercept only when at or below the published glideslope intercept altitude and outside the final approach fix. Confirm the GS is captured from below, with the pointer coming down to centre, not down from the top of the scale.
- If you cannot re-intercept before the FAF, fly the missed approach and re-plan. Do not attempt the approach from above the path.
Report the event. A false capture at that height also suggests you were outside the transmitter's normal glideslope coverage, which is worth noting for the operator and the ATS unit.
3. What cues would have detected this earlier, and what would you brief differently next time?
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Cues to catch it earlier:
- Know the profile. Brief the chart's altitude at each DME/waypoint - about 2,090 ft at 6.5 DME for a 3° path plus threshold elevation - before intercept.
- Intercept from below. An approach clearance while high is not an instruction to dive; ask for descent early or a hold.
- Monitor rate of descent. Roughly 5 × groundspeed in knots gives the approximate 3° rate (200 kt ≈ 1,000 ft/min). A demand of three times that is a red flag.
- Watch the capture. A pointer that centres while you are far above profile, or captures outside normal coverage, is suspect.
- Use the aural warnings. GPWS/EGPWS 'GLIDESLOPE' and 'SINK RATE' alerts and the terrain display are independent cross-checks.
- If unsure, cross-check DME against altitude, level off and sort it out before the FAF.
Next time: brief the intercept altitude, the expected rates of descent and the go-around trigger, and treat any approach clearance given while significantly above profile as a prompt to negotiate, not to comply.